The present invention relates to a copier, facsimile apparatus, laser printer or similar digital image forming apparatus and, more particularly, to an image forming apparatus capable of producing high quality halftone images.
Today, it is a common practice with a computer to use an image output unit implemented as a laser beam printer or similar electrophotographic image forming apparatus. This kind of image forming apparatus has sharply extended a desktop publishing field due to various advantages available therewith, e.g., high image quality and low noise. At the same time, in parallel with the increasing memory size, processing speed and functions and decreasing cost of a controller used to control, for example, a laser beam printer, there has been extensively used a method capable not only of printing out data in black and white, i.e., in two levels, but also of converting multilevel image data to bilevel image data by a dither method or a density pattern method and then printing out the bilevel data. A pulse width modulation (PWM) system has been recently proposed as an advanced approach which implements resolution and halftone reproducibility simultaneously at a higher level. The PWM system is advantageous over the bilevel dither method and density pattern method in that it reduces the matrix size for halftone having a given tone value. Specifically, an PWM type image forming apparatus receives image data in the form of a multilevel signal and drives a laser by a pulse width matching the level of the signal, thereby providing a single dot with tonality. However, the problem with the PWM scheme is that several dots have to be subjected to PWM at the same time, resulting in low resolution even when characters or lines are printed.
In light of the above, there has also been proposed a printer capable of printing each of a photograph or similar halftone image (multilevel image) and a character or similar text image (bilevel image) in different resolution. The kind of printer prints a bilevel image in a resolution of 300 dots per inch (dpi) or prints a multilevel image in a resolution of 150 dpi by PWM. All the bilevel images are written to a bilevel image memory while multilevel images are drawn in a multilevel image memory, and then they are output together in a superposed condition. This renders both the bilevel (or text) portions and the multilevel (halftone) portions with high quality. However, With this type of conventional device, the difference in resolution between the images brings about a problem when a text image should be superposed on a halftone image, when a halftone image should be partly removed to represent a text image, or when a text image should be filled with a halftone image (i.e. a halftone image should be clipped by the contour of a text image). In such a case, the contour of a character or the boundary between a character and a halftone image is accompanied by an unexpected space or appears unnatural to the viewer's eye.